My lemon tree is wilting. What should I do?

I understand that raising a lemon tree from a seed is the hardest thing to do in most climates. However, I have been successful in raising it to be a small plant about six inches tall in a small 8 inch pot. But a couple of weeks ago, my lemon tree started to wilt. I have done everything I can from providing it with water, sun, even moving it away from any heat vent. It is still too earily to put it outside (being January in St. Louis), so what can I do?